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Method

Preheat the oven to 180°C, gas mark 4. Weigh the beef and calculate the cooking time using our guidelines below.

Remove the thyme leaves from the stalks and pulverize with the pepper and oil using a pestle and mortar. Rub this mixture all over the beef. Place the joint in a large roasting tin with the garlic cloves alongside and roast for the calculated time, basting occasionally.

After about 40 minutes the garlic should be very soft. Remove the garlic cloves from the tin and allow to cool slightly, then squeeze the cloves from their skins into the mayonnaise. Mix well and place in a serving bowl. Cover and chill until ready to serve.

Now, cut each sweet potato into thick chips. About 40 minutes before the beef is ready, drain all but about 2 tablespoons of fat from the tin, discarding or reserving. Then add the chips to the roasting pan, tossing to coat. Return the tin to the oven. Turn the chips once during the remaining cooking time.

Remove the beef from the oven and place on a carving board. Cover loosely with foil and leave to rest for 10-15 minutes. Increase the oven temperature to 200°C, gas mark 6, and return the chips to the oven for a further 10-15 minutes until golden.

Carve the beef thinly and serve with the sweet potato chips, a dollop of garlic mayonnaise and a helping of Waitrose Peas, Leeks & Spinach Medley or Waitrose Purple Sprouting Broccoli Medley.